The invention relates generally to electronic circuits, and more particularly to an integrated device that routes a signal through the device and to other devices in an integrated-circuit module.
In today""s marketplace, consumers are pressuring manufacturers of electronic products to squeeze more functionality into a smaller space. For example, consumers want smaller and lighter personal computers, such as laptops, that have faster, more powerful processors and greater memory capacity.
FIG. 1 is a side view of an integrated-circuit (IC) module 10, which allows manufactures to reduce the circuit-board area, and thus the overall size, of electronic products such as personal computers. The module 10 includes a number of integrated devices 12, which are stacked one atop the other, and is mounted to a circuit board 14. Therefore, no matter how many devices 12 it includes, the module 10 occupies the circuit-board area of only one device 12. This is a significant reduction in occupied area as compared to a scheme where the devices 12 are laid out side by side on the board 14.
More specifically, each of the devices 12 in the module 10 has a conventional package that allows coupling of signals between the board 14 and all of the devices 12. In the illustrated embodiment, the devices 12 each have a ball-grid-array (BGA) package, although other packages may be used as long as they allow stacking of the devices 12 to form the module 10. Each device 12 includes a number of connection balls 16, which are each coupled to a respective terminal 18. A respective conductor 19 couples each of the terminals 18 to a respective terminal 20 that is aligned with the terminal 18. For example, in the device 120, the conductor 190 couples the terminal 180 to the terminal 200. When the devices 12 are stacked to form the module 10, respective conductive paths are formed by the connection balls 16, the terminals 18 and 20, and the conductors 19. It is these conductive paths that couple respective signals between the circuit board 14 and all of the devices 12 in the module 10. For example, one such conductive path is formed by the ball 160, terminal 180, conductor 190, terminal 200, ball 161, and so on up to the terminal 20n. Therefore, so that the module 10 works properly, all of the devices 12 have the same pin out, i.e., receive the same signals on the same respective terminals 18 and provide the same signals on the same respective terminals 20.
Unfortunately, referring to FIG. 2, which is a top view of one of the devices 12 of FIG. 1, the size of each device 12 is increased to accommodate signals that are not common to all of the devices 12. For example, each device 12 is enabled by a respective chip-select signal CS0-CSn, which is received on a respective chip-select terminal 18CS0-18CSn. If they were laid out side by side on the board 14 (FIG. 1), then each of the devices 12 would need only one chip-select terminal 18CS. But because they are stacked, each device 12 has the same number of chip-select terminals 18CS0-18CSn as there are devices 12 in the module 10 (FIG. 1).
More specifically, for each unique signal such as a chip-select signal that they receive, the devices 12 each need n terminals, where n is the number of devices 12 in the module 10 (FIG. 1). Thus, just one or two unique signals may cause a significant increase in the sizes of the devices 12. For example, the device 120 (FIG. 1) responds only to CS0, and thus needs only the terminal 18CS0 to function properly. That is, the device 120 has no need for CS1-CSn, and thus can function properly without the terminals 18CS1-18CSn. But because the other devices 121-12n in the module 10 respond to CS1-CSn, respectively, the device 120 must also include the terminals 18CS1-18CSn to form conductive paths that couple CS1-CSn to the devices 121-12n. For reasons including that the relative position of a device 12 in the module 10 is unknown during manufacture of the device 12, each of the devices 121-12n also includes a respective set of terminals 18CS0-18CSn.